perf data ctf: Add value_set_string() helper

There are many value_set_##x helper for integer, but only for integer.
This patch adds value_set_string() helper to help following commits
create string fields.

Signed-off-by: Wang Nan <wangnan0@huawei.com>
Acked-by: Jiri Olsa <jolsa@kernel.org>
Cc: Zefan Li <lizefan@huawei.com>
Cc: pi3orama@163.com
Link: http://lkml.kernel.org/r/1466767332-114472-2-git-send-email-wangnan0@huawei.com


Signed-off-by: Arnaldo Carvalho de Melo <acme@redhat.com>